In recent years, interest has grown in the development of intense sources of far infrared (FIR) laser radiation for applications such as plasma diagnostics based on collective scattering of radiation by electron density fluctuations in plasmas of long Debye length. See, for example, D. E. Evans et al., "Far Infrared Superradiant Laser Action in a Methyl Fluoride", Applied Physics Letters, Volume 26, Number 11, June 1, 1975, at page 630, and D. E. Evans et al., "Far Infrared Super-radiant Laser Action in Heavy Water", Optic Communications, Volume 18, Number 4, September, 1976, at page 479.
Unstable resonators have been used for several years as a means of extracting high power, low divergence pulses from lasers with large Fresnel numbers. A detailed discussion of the characteristics of unstable optical resonators is given in Siegman, a.e., Applied Optics, Volume 13, Number 2, February, 1974, beginning on page 353, and a more general discussion is provided in Siegman, a.e., An Introduction to Lasers and Masers, McGraw Hill Book Company, 1971, at pages 342 and 343.
The conventional stable resonator, whose mirror configuration corresponds to a stable periodic focusing system, has a long slender Gussian profile lowest order mode whose diameter is on the order of a few times (L.lambda.).sup.1/2, where L is the cavity length and .lambda. is the wavelength, which is generally less than the diameter of the laser mirrors themselves. If the diameter of the laser medium or tube is 2a, where a is the maximum transverse beam diameter, then the area ratio of the laser medium cross section to the lowest mode cross section is of the same order as the Fresnel number N.sub.F =(a.sup.2)/(L.lambda.) characterizing the laser medium. If this Fresnel number is much larger than unity, the lowest order mode will extract only a fraction (about (1)/(N.sub.F)) of the energy available in the laser medium. The laser must thus oscillate in a sizeable number of the higher order modes to extract all the energy from the laser medium.
The lowest order mode in the unstable resonator, by contrast, since the unstable resonator corresponds to a divergent periodic focusing system, expands on repeated bounces to fill the entire cross section of at least one of the laser mirrors, however, large it may be. The laser output is taken as a diffraction coupled beam passing around rather than through the output mirror. A more detailed discussion concerning the differences between stable and unstable optical resonators is given in the Siegman text, supra, at Chapter 8.
Unstable resonators have not yet been widely used as pump lasers for far infrared systems because the intermixing of spherical and plane wavefronts makes them difficult to tune. One approach that has been taken to tune an unstable resonator is to inject the beam from a grating tuned CW laser to seed the oscillation on the desired wavelength, using an intracavity absorption cell where necessary to suppress unwanted strong transitions. See, for example, D. P. Hutchinson et al., Applied Optics, Volume 16, page 293 (1977). A more direct tuning technique is required, however, if weaker lines are to be made accessible while retaining the large mode volume and excellent transverse mode discrimination possible with unstable resonators.
One object of the present invention, therefore, is to provide a new and improved method of and system for narrow line tuning of a high power CO.sub.2 pump laser.
Another object is to provide a new and improved unstable resonator that is narrow line, grating tuned.
Another object is to provide an unstable resonator that is directly tunable to weak lines while retaining large mode volume and high quality transverse mode discrimination.
Another object is to provide a grating tuned unstable resonator that has reduced sensitivity to extracavity feedback normally associated with unstable resonators.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.